Two methods for winding magnet coils are known. In the “mandrel method”, the winding form is first placed onto a spike or mandrel, which is then rotated together with the winding form so that conductor wire is wound around the winding form from the supply of conductor wire. In the “winding with flyer” method, the winding form is held stationary whereas the supply of conductor wire is rotated around the winding form, with the effect that the conductor wire is wound around the winding form.
Regardless which one of these methods is used, winding of a coil on a winding form, the diameter of which changes stepwise across the winding form, has to be started at the position where the winding form diameter is at smallest, and then the coil must be wound, layer for layer, until the desired winding height has been achieved in order to ensure a smooth and controllable winding. Otherwise, an effect as shown in the series of FIGS. 1 to 3 may occur.
A prior art winding form comprises a first part 18 and a second part 10, and a step 15 between them. The first part 18 has a larger diameter than the second part 10, and both are limited by respective end walls 11, 12. If winding of such a winding form is started on the first part 18, the conductor wire 13 can be wound, starting from the left hand side of FIG. 1, with no problem only until the step 15. When the winding goes beyond the step 15 to the right (FIG. 2), however, the conductor wire 13 that is being wound on the side 10 of the winding form having a smaller diameter pulls down some of the conductor wire 13 already wound on the other side 18 of the step 15. Since the winding is usually performed so that the conductor wire 13 is under tension, some conductor wire on the side 18 with a larger diameter slips down almost unavoidably. This may easily result in a tuft of conductor wire at the end position L of the step 15, making the resulting coil useless.
US 2005/0040725 A1 discloses a bobbin that comprises a hollow-cylindrical middle section and two lateral hollow-cylindrical body sections. The middle section has its diameter diminished compared to the two lateral body section thus forming an annular recess which allows a magnet wire to be wound with an additional number of turns around the bobbin.
The English abstract of EP 0 070 752 discloses a security transformer, a first winding of which is located within a second winding and the integral bobbin of which has a cylindrical portion about which the internal winding is wound and which is extended, at each of the axial ends, by cylindrical portions of larger section forming supports of an insulating foll surrounding the internal winding and whereabout the external winding is wound, the non-insulated connection conductors of the internal winding crossing a cylindrical end portion. Grooves are arranged within a cylindrical end portion, extending along a spiral path prolonging the spiral of the internal winding, and devices are provided for applying these non-insulated conductors into the bottoms of the grooves.
To avoid slipping of the conductor wire at the step, all manufacturers, if willing to wind a coil on a winding form whose diameter changes across the winding form over a step, need to start the winding from the part of the winding form that has the smallest diameter.